Consumers demand value for money and purchase decisions are affected by the appearance of a product and its quality. The display of products in so-called blister, bubble or clamshell packages is common for different classes of consumable goods.
Blister packs are most often the secondary packages for a further primary package held within, however for items such as electrical cables, some computer accessories, food products, and so-forth, the blister or clamshell pack itself is the only package. In any case, blister packs enable the consumer to see and appreciate the product inside and inspect it. If the blister package were soiled or stained internally in an unsightly manner, this would invariably affect the consumer's decision to buy the product, as the product would appear defective.
Cyanoacrylate (CA) adhesives, or so-called superglues, are popular consumer goods. Cyanoacrylate is the generic name for a family of resistant fast acting adhesives based on esters of 2-cyanoacrylic acid. The structure of the monomer is as follows:
wherein R is usually an alkyl group such as, for example, methyl, ethyl, butyl, or octyl, or an alkoxyalkyl group, for example, 2-methoxymethyl or 2-ethoxyethyl.
Such compounds have been well known for some time, as described in, for example, S. Ebnesajjad Ed., Adhesives Technology Handbook, William Andrew, Norwich, 2008.
In many industrial and domestic applications CAs are used in form of one-component, as they polymerize rapidly when they are in the form of a thin film between two substrates in the presence of anions or nucleophilic species. The speed at which the bond is formed and the ease of use have contributed to their popularity. For other applications CAs are used in form of two components, one component containing the CA and the other a composition comprising plasticizer and catalyst, or a non-CA polymerisable monomer mixture and a catalyst. These 2K CA adhesives show high technical performance characteristics.
One-component CAs are sold in various types of primary packages most commonly in bottles or aluminium tubes. Such packs are usually subsequently contained in secondary packaging such as foldable boxes, or in blister packs. The moisture sensitive nature of these very reactive adhesives necessitates that the primary package is tightly closed by a removable cap for bottles, or a membrane seal for tubes as disclosed, for example, in International patent applications WO-A-99/44907, WO-A-01/56894, WO-A-2005/075312, and WO-A-2007/004203. This current packaging method is essential for all CA based adhesives irrespective if they are known to emit vapours or not.
The problem of vapour emission from common one-component CA based adhesives and the staining it causes, has heretofore not been encountered in blister packs containing these traditional hermetically sealed primary packages when the products are first displayed. That staining is known as ‘blooming’ or chlorosis, and it is disclosed, for example, in the British patent application GB-A-2067553.
More recently so-called two-part, two-component or 2K, cyanoacrylate adhesives have appeared in the marketplace. Whereas the concept of 2K CAs is well known in the prior art, as disclosed, for example, in U.S. Pat. No. 3,282,773, they have only emerged as articles of commerce recently and in double-barrelled syringes that are used as primary packages, for example, Loctite 3090, Loctite 3092, Loctite 4090, Afinitica Super Repair, Afinitica Soldadura Adhesiva, or Afinitica Soldadura Adhesiva Metal.
To date such syringes are most usually contained in foldable boxes that do not allow the product to be visible. However, one such product, Loctite 3092, has been commercialized using a semi-transparent plastic bag as a secondary package. White staining on the inside of the aforementioned plastic bag is evident in that case and detracts from the appearance of the product.
The problem arises because the CA adhesive contains a monomer constituent that has a high vapour pressure. Common CA monomers, such as alkyl CAs, and in particular ethyl cyanoacrylate (ECA) are known to have high vapour pressure, be lachrymatory and irritant in nature and have product labelling to indicate this fact. The syringe package format containing such CAs is not hermetically sealed. Whereas the syringe format for 2K CAs has a tightly fitting re-closable cap at the dispense orifice end (proximal end), its base is movable by virtue of a snug-fitting piston located within syringe barrels. The product is confined between the piston and the proximal end of the syringe. Such 2K CA products are most commonly of a gel consistency and the snug-fitting piston confines the bulk product well.
Although pistons in syringes may be considered to confine ECA based bulk compositions in good condition, they cannot retain emissive vapours issuing from the bulk product since vaporized molecules easily pass non gas-tight parts. This phenomenon creates a significant specific problem uniquely when presenting syringe based ECA products in transparent blister packs because the vapours polymerise inside the blister bubble and manifest as an unsightly white stain. This staining creates the impression of a defective product and influences purchasing decisions by consumers. On the other hand, this very staining feature of common CAs is used to advantage in unrelated applications and they are materials of choice for the visualization of fingerprints in forensic science.
In the case of syringes containing a CA composition, the piston must move during product dispensing and, thus, a small but finite gap exists between the walls of the syringe barrel and the piston element itself. Furthermore, because CAs are such effective adhesives, the pistons in these syringes do not employ deformable O-rings to seal finite gaps because the latter become glued to the container walls and lock.
This piston-stick problem was identified in the prior art and a solution was proposed in U.S. Pat. No. 5,016,784 by use of hydrocarbon grease disposed between a non-stick polymeric seal and a CA adhesive contained in a syringe barrel. Thus the grease was in direct contact with the CA adhesive. This grease aids smooth advancement of the plunger (piston) in the barrel. This alternative solution proposed specifically for single-barrelled CA syringes, creates significant problems for mass production in the case of double-barrelled syringes for the recently commercialised 2K CAs. Before filling, the pistons in each of the two barrels in a 2K empty syringe are inserted and pushed to the proximal end to a position above the orifices. The adhesive part (so-called Part A) and the catalyst-plasticizer, or catalyst-non-CA monomer mixture part (so-called part B) are simultaneously pumped into the respective syringe barrels through the individual orifices under pressure to fill the chambers of the syringes in an operation that also forces the pistons back into their ultimate final position which is intermediate along the length of the syringe. The technical solution of the '784 patent, when applied to a double-barrelled syringe, implies that the hydrocarbon grease would have to be first introduced into the barrel before either product part in a separate operation and also must be introduced in both chambers so the pistons rise up to the same position in each chamber to ensure subsequent proper dispensing from the double plungers supplied with double-barrelled syringes which have equal plunger lengths. Compatibility of hydrocarbon grease with plasticizers and catalysts is not foreseen in the '784 patent, which is addressed to a single component product. Furthermore, due to the special design described in the '784 patent, the syringe requires an end-stop within the barrel to prevent final discharge of the hydrocarbon grease, which is an ‘anti-adhesive’, and a significant air gap is formed between the greased piston in the stopped position and the orifice as illustrated in FIG. 3 of that US patent. That situation would be amplified twofold if applied to a 2K CA syringe and such included air pockets would generate serious problems in uniform dispense and mixing of a 2K CA product. Those problems are not anticipated in a single-barrelled syringe. In fact and by default, compositions suited to single-barrelled syringes do not require mixing during dispense unlike those from double-barrelled syringes, which require uniform mixing of the CA and the catalyst. The mixing of components of compositions is thus unique to 2K CA syringes and the solution proposed by the '784 patent to prevent piston-stick, is not practical for 2K CA syringe design, end use, or for their filling in mass production.
Another approach is selling 2K CAs in a foldable box. In that case the problem is concealed, but still exists. However there is no initial negative impact on the customer unless the box is opened. On the other hand, the advantage of allowing a customer to directly examine the product for purchase is forgone.
Lastly, many patents have detailed the design of 2K syringes and their intricate components, as disclosed in, for example, US Patent Application US-A-2013/0177870, International patent applications WO-A-2005/075312 and WO-A-2006/005206, and European patent application EP-A-0730913, especially with particular attention to the design of securely sealing the syringes, for example, with special re-closable caps for the orifice (proximal) end.
However no consideration has been directed to problems regarding the holistic quality of display package containing piston-sealed syringes comprising products with volatile constituents that cannot avail of O-rings on pistons.
There is, thus, an urgent need to provide a corrective, cost effective, and industrially easy-to-implement solution to the packing of 2K CA syringes that contain volatile CA constituents. Aside from creating marketing appeal and visibility of a product, a further rationale for use of blister packaging is to reduce cost so that a solution to the problem should be realized using commercially available syringes rather than specially designed ones with internal end-stops to prevent exudation of anti-adhesive materials such as greases at any time during product use.